1. Field of the Invention
This invention relates to nuclear magnetic resonance (NMR) apparatus.
NMR apparatus of the kind capable of performing chemical analysis of a body by spectroscopy have been known for many years. More recently NMR apparatus have been developed which are of a kind capable of imaging a body to provide a representation of the distribution, over a selected cross-sectional slice or volume of the body, of a chosen quantity, for example the density of chosen nuclei such as hydrogen protons or of NMR spin relaxation time constants.
2. Description of Related Art
In some applications it would be useful to be able to perform the chemical analysis of a body, and the imaging of the body on the same NMR apparatus. In practice, however, whilst both kinds of NMR apparatus described above require a magnet system capable of providing a static magnetic field which is used to define the equilibrium axis of magnetic alignment of the nuclei within the body, the static magnetic fields which are required in the two different kinds of apparatus for optimum performance are different. Thus, in an NMR apparatus to be used for both chemical analysis and imaging techniques it is necessary to adjust the magnet system to produce a different field strength for use in each technique. The requirement for adjustment of the magnet system presents several practical difficulties, as adjustment of the magnetic field strength necessitates the realignment of the rest of the apparatus which is both time consuming and difficult. These difficulties are exacerbated where a cryogenic magnet system is being used as such as magnet system should ideally be allowed to run uninterrupted.